An anesthetic ventilating apparatus having a closed breathing circuit is disclosed in European Patent Publication 0 121 255. This anesthesia ventilating apparatus utilizes a complex control loop for metering anesthetic gas in which the anesthetic gas components are sensed by means of appropriate sensors and the sensor signals are utilized for driving a metering unit. At any time during a ventilation, the fill level of the breathing gas in the breathing circuit is determined and the required fresh gas quantity is supplied.
The control loop for metering the anesthetic gas is essentially conceived for maintaining a quasi-stationary operating condition; that is, the previously adjusted concentration values of the anesthetic gas components are maintained pursuant to a determined unchangeable control algorithm and only as much gas is metered as was consumed.
It is a disadvantage with the known anesthesia ventilating apparatus that for a change of the concentration proportion of individual anesthetic gas components such as the anesthetic agent concentration, new values are reached in part only after substantial adjusting times with the measured concentration in the breathing circuit approximating the new desired value asymptotically. This is for many applications intolerable such as for the transition from the induction phase with higher anesthetic agent concentration to the maintenance phase with lesser concentration.
An anesthesia ventilating apparatus having controlled metering of the anesthesia agent vapor is disclosed in the British Journal of Anaesthesia (1983), 55, pages 1065 to 1075. The breathing system supplying the patient with breathing gas is connected to a ventilator for assisted ventilation and is supplied via a gas metering unit with the anesthetic gases: nitrous oxide, oxygen and anesthetic agents. A central microprocessor controlled control unit on the one hand influences the anesthetic agent metering unit in the manner of a desired-value generator and, on the other hand, registers the actual value which is measured with an anesthetic agent sensor in the breathing system downstream of the patient. When adjusting to a new anesthetic agent desired value, the control unit first supplies an actuating-variable signal to the metering unit which is greater by a multiple than the new desired value which is to be set. This first phase takes approximately nine respiratory cycles and has the purpose of determining parameters specific to the system from the spontaneously adjusting anesthetic agent concentration change in the breathing system. These parameters and further constants stored in the control unit are combined with each other in order to bring the anesthetic agent concentration in the breathing system closer to the new desired value in a stepwise manner during a second phase which has a duration of approximately 90 respiratory cycles. In the third phase, the anesthetic agent concentration is adjusted to the selected desired value by switching in a controller.
It is a disadvantage with this anesthesia ventilating apparatus that a new desired value for the anesthetic agent concentration is adjustable only after a complex measuring and computing program wherein constants from a table have to be considered and inputted to a control unit in advance. The division into three phases is impractical for the clinical routine. Furthermore, there is no direct coupling between the parameters measured in the first phase and the adjusted values of the anesthetic agent controller which is switched in in the third phase.